An antigen is any foreign substance that results in an immune response. Our immune systems are great at recognizing things that are not supposed to be there. This is done in a number of ways, but one of them is by producing antibodies that very specifically recognize an antigen. Antibodies themselves don’t kill viruses or the cells that have been infected by a virus, but they do recognize them, essentially flagging them for destruction by other immune cells.
An antigen test can be designed to detect any viral piece. The antigen test used for SARS-CoV-2 detects the virus’s capsid protein. The test itself uses antibodies that specifically bind the SARS-CoV-2 capsid protein that is secured to the test strip.
The test is then exposed to the mucus sample collected from the nose or throat. If the viral capsid is present in the sample, it will bind the antibody, producing a signal that can be read by the lab personnel.
These tests resemble pregnancy tests, with a control band and a second band appearing in samples that are positive for the viral capsid.